ABSTRACT This proposal requests partial support for travel expenses for U.S. experts to attend the Fourth International Medical Symposium on Erdheim-Chester Disease to take place in Paris, France. Erdheim-Chester disease (ECD) is an rare histiocytic disorder with approximately 600 cases reported world-wide since its initial description in 1930, although the true incidence is not known as many cases are unreported. As will be outlined in this proposal, international collaboration has been vital to major advances in the understanding and therapy of ECD. The symposium will take place at the Piti-Salptrire Hospital, where the largest cohort of ECD patients (150) is treated. The ECD Global Alliance (ECDGA) has sponsored three International Medical Symposia on ECD to bring together junior and senior experts, and each of these meetings has led to multi- institutional collaborations with high-quality publications in significant journals. The specific aims of this symposium will to be convene over 40 basic scientists and clinical experts in ECD to present and synthesize current research findings and clinical experience in ECD. Moreover, this meeting will have the concrete and deliverable aim of developing criteria for treatment response to be implemented in future clinical trials of ECD. The meeting will take place over one day and will have sessions for oral presentations about basic science, genotyping efforts, and collective clinical experiences. There will two dedicated sessions about modalities to evaluate treatment response and a consensus discussion about response criteria. The significance of this meeting is that it will bring together these experts with a track record of productive collaboration and consensus work for a new, distinct, and achievable goal. This symposium is imminently health-related as it impacts clinical care of ECD patients worldwide and will define response evaluation for emerging ECD clinical trials. This annual meeting is critical for ongoing fruitful international collaboration about the latest ECD science and its clinical application.